Recently, radio frequency identification (RFID) technology has gained tremendous popularity as a device for storing and transmitting information. RFID technology utilizes a tag transponder, which is placed on an object, and a reader, also referred to herein as an interrogator, to read and identify the tag. RFID technologies are broadly categorized as using either “active” tags or “passive” tags. Active tags have a local power source (such as a battery) so that the active tag sends a signal to be read by the interrogator. Active tags have a longer signal range. “Passive” tags, in contrast, have no internal power source. Instead, passive tags derive power from the reader, and the passive tag re-transmits or transponds information upon receiving the signal from the reader. Passive tags have a much shorter signal range (typically less than 20 feet).
Both categories of tags have an electronic circuit that is typically in the form of an integrated circuit or silicon chip. The circuit stores and communicates identification data to the reader. In addition to the chip, the tag includes some form of antenna that is electrically connected to the chip. Active tags incorporate an antenna that communicates with the reader from the tag's own power source. For passive tags, the antenna acts as a transducer to convert radio frequency (RF) energy originating from the reader to electrical power. The chip then becomes energized and performs the communication function with the reader.
On the other hand, a chipless RFID tag has neither an integrated circuit nor discrete electronic components, such as the transistor or coil. This feature allows chipless RFID tags to be printed directly onto a substrate at lower costs than traditional RFID tags.
As a practical matter, RFID technology uses radio frequencies that have much better penetration characteristics to material than do optical signals, and will work under more hostile environmental conditions than bar code labels. Therefore, the RFID tags may be read through paint, water, dirt, dust, human bodies, concrete, or through the tagged item itself. RFID tags may be used in managing inventory, automatic identification of cars on toll roads, security systems, electronic access cards, keyless entry and the like.
Antennas are an element of RFID tags that are typically prepared via stamping/etching techniques, wherein a foil master is carved away to create the final structure. The RFID antenna may also be printed directly on the substrate using a conductive metal ink. The ink is printed on a substrate, followed by high temperature sintering in order to anneal the particles and to create a conductive line on the substrate. Alternatively, metal fibers may be incorporated directly into the substrate. For example, one chipless RFID technology from Inkode Corp® uses embedded aluminum fibers that are embedded into paper. The aluminum fibers must be cut to the appropriate wavelength (¼ wavelength) and be incorporated into the paper fibers as a furnish additive during the papermaking process. Therefore, the Inkode® method is costly and tedious.
Although particulate metal materials may be used, the superior characteristics of nanoparticle metal materials in ink applications yields a better product. Metallic nanoparticles are particles having a diameter in the submicron size range. Nanoparticle metals have unique properties, which differ from those of bulk and atomic species. Metallic nanoparticles are characterized by enhanced reactivity of the surface atoms, high electric conductivity, and unique optical properties. For example, nanoparticles have a lower melting point than bulk metal, and a lower sintering temperature than that of bulk metal. The unique properties of metal nanoparticles result from their distinct electronic structure and from their extremely large surface area and high percentage of surface atoms.
Metallic nanoparticles are either crystalline or amorphous materials. They can be composed of pure metal, such as silver, gold, copper, etc., or a mixture of metals, such as alloys, or core of one or more metals such as copper covered by a shell of one or more other metals such as gold or silver. The nozzles in an inkjet printing head are approximately 1 μm in diameter. In order to jet a stream of particles through a nozzle, the particles' size should be less than approximately one-tenth of the nozzle diameter. This means that in order to inkjet a particle, its diameter must be less than about 100 nm.
Nickel has been used for conductive inks for a very limited extent because of its relatively low conductivity (approximately 4 times less than that of copper or silver). Gold and silver can provide good conductivity, but are relatively expensive. Moreover, gold and silver require high temperatures for annealing, which can pose a challenge for printing on paper and plastic substrates. Copper provides good conductivity at a low price (about one percent of that of silver). Unfortunately, copper is easily oxidized and the oxide is non-conductive. Conventional copper-based nanoparticle inks are unstable and require an inert/reducing atmosphere during preparation and annealing in order to prevent spontaneous oxidation to non-conductive CuO or CU2O. Copper polymer thick film (PFT) inks have been available for many years and can be used for special purposes, for example, where solderability is required. Another interesting strategy is to combine the advantages of both silver and copper. Silver plated copper particles are commercially available, and are used in some commercially available inks. Silver plating provides the advantages of silver for inter-particle contacts, while using the cheaper conductive metal (copper) for the bulk of the particle material. Thus, the only reliable means of preparing copper antennas is via electroplating on an existing metal surface.